As a background for the recent increase in communication traffic, the demand for optical communication/transmission apparatuses is increasing. Not only for optical repeating nodes introduced with backbone networks, but also recently, the introduction of optical transmission apparatuses for local networks is being actively performed. Furthermore, optical networks are also being formed for subscriber loops. In this manner, optical communication systems bear an important role with respect to world information networks. Therefore, naturally, high dependability is required for optical communication systems.
As an important means for maintaining high dependability of optical communication systems, there is an information transmission function using optical supervisory channel (OSC). The OSC light is transmitted on a transmission line together with an optical communication signal (main signal light), and by transmitting operation information or performance information for an optical communication system to various parts in the optical communication system, maintenance of the transmission characteristics and smooth handling in the event of problems are realized. In conventional optical communication systems, normally, as illustrated in FIG. 11, OSC light Losc is arranged in a wavelength band separated from a wavelength band where a plurality of main signal lights Ls of different wavelength are arranged.
As a general system configuration for transmitting and receiving such OSC light Losc between optical transmission apparatuses, a configuration for example as illustrated in FIG. 12 is well known, where in a transmission side optical transmission apparatus 110, the OSC light Losc generated by an OSC transmitter 112 passes through a multiplexing filter 113 provided on the output side of a main signal optical amplifier 111 and is multiplexed with the main signal light Ls and transmitted to a transmission line 101, and the OSC light Losc transmitted to the transmission line 101, is separated in a reception side optical transmission apparatus 130, from the main signal light Ls by a demultiplexer 131 provided on the input side of a main signal optical amplifier 132, and received by an OSC receiver 133.
In the above optical communication system which uses OSC light, if the repeating distance between the optical transmission apparatuses becomes long, the losses of the transmission line increase. More specifically, the loss per unit length of the transmission line is generally around 0.2 dB/km, and the loss of the transmission line for one repeating section increases corresponding to the repeating distance. Furthermore, in the case where various functional optical components are arranged on the transmission line, the transmission losses of these functional optical components add up so that the repeating losses become even greater. Therefore, as the light level of the transmission light reaching the reception side becomes smaller, the transmission characteristics deteriorate, so that there is a likelihood of an increase in the number of reception errors per unit time. In particular, for the aforementioned OSC light arranged in the wavelength band as illustrated in FIG. 11, the loss of the transmission line is greater than for the main signal light. Furthermore, since this also receives an influence from the Raman effect of the main signal light existing on the long wavelength side, a decrease in the light level after transmission is likely to occur.
In order to avoid the aforementioned transmission characteristic deterioration accompanying the increase in the long distance of the repeating section, then for the main signal light Ls, in the configuration of FIG. 12, this can be dealt with by increasing the gain (optical output power) of the main signal optical amplifier 111 on the transmission side. Furthermore, even when coping in this way, in the case where the light level of the main signal light Ls after transmission is insufficient, it is also effective to apply transmission line distributed Raman amplification (DRA) such as known from the document; M. Takeda et al., “Active Gain-Tilt Equalization by Preferentially 1.43 μm- or 1.48 μm-Pumped Raman Amplification”, OAA '99, ThA 3-1, 1999, and add a DRA unit 150 for example as illustrated in FIG. 13, that supplies pump light Lp to the transmission line 101, and then Raman amplify the transmission light using the amplification effect due to the induced Raman scattering effect. In this case, in the DRA unit 150, by providing a configuration for Raman amplifying the OSC light Losc (pump light sources (LD) 151C, multiplexers 152C and 153C) in addition to a configuration for Raman amplifying the main signal light Ls (pump light sources (LD) 151A and 151B, multiplexers 152A, 152B, 153B, and 154), it is possible to suppress a drop in the reception level not only of the main signal light Ls but also of the OSC light Losc.
Incidentally, in the above described optical communication system to which is applied the main signal light optical amplifier or the optical amplifying device such as a DRA unit, unnecessary noise light other than the main signal light Ls and the OSC light Losc is generated in the optical amplifying device. Furthermore, when the optical amplifying device is connected in multi-stages, the above noise light accumulates, and hence the power of the noise light reaching the reception end becomes large, so that there is a problem in that the transmission characteristics of the main signal light Ls and the OSC light Losc are deteriorated.
As a conventional technique for reducing the influence of the noise light generated by the optical amplifying device as mentioned above, for example as illustrated in FIG. 14, there is proposed a configuration where an optical circulator 202 and a fiber Bragg grating 203 that reflects the OSC light Losc, are provided on the input end of an optical fiber amplifier 201, and the OSC light Losc that is reflected by the fiber Bragg grating 203 and taken out by the optical circulator 202 is received by an OSC receiver 205 via an optical filter 204, and in the optical filter 204, spontaneous emission light that is generated by the optical fiber amplifier 201 such as an EDFA and propagated in an opposite direction to the main signal light Ls is cut off (refer for example to Japanese Laid-open Patent Publication No. 2000-224116).
However, in the above conventional configuration as illustrated in FIG. 14, while this reduces the influence of the opposite direction spontaneous emission light (noise light) from the optical fiber amplifier 201 on the OSC light received by the OSC receiver 205, there is a problem in that the influence of the noise light propagating in the same direction as the main signal light and the OSC light, generated on the upstream side from the optical fiber amplifier 201 is not reduced. That is to say, the spontaneous emission light that is cut off by the optical filter 204 is light that has passed through the fiber Bragg grating 203, and hence the wavelength band is different to the wavelength band of the OSC light. Therefore, regarding the noise light that is generated on the upstream side from the optical fiber amplifier 201, the wavelength component corresponding to the wavelength band of the OSC light is reflected by the fiber Bragg grating 203, and passes through the optical circulator 202 and the optical filter 204, and is input to the OSC receiver 205, so that the reception characteristics of the OSC light are deteriorated. In particular, in a system to which Raman amplification of OSC light is applied, the noise light generated in the Raman amplification is added to the spontaneous emission light generated in the optical fiber amplifier on the upstream side. Therefore regarding the input light to the OSC receiver 205, the proportion of the OSC light power with respect to the noise light power becomes small, so that there is a possibility that the count of reception errors per unit time increases.